A polymer electrolyte fuel cell in which a hydrogen-containing fuel gas and oxygen-containing oxidizing gas are supplied to an anode and cathode, respectively, and an electromotive force is generated by an electrochemical reaction occurring at both poles is generally constituted by sequentially laminating a bipolar plate, a gas diffusion electrode substrate, a catalyst layer, an electrolyte membrane, a catalyst layer, a gas diffusion electrode substrate, and a bipolar plate. The gas diffusion electrode substrate is required to have high gas diffusivity for allowing a gas supplied from the bipolar plate to be diffused into the catalyst layer and high water removal performance for discharging water generated by the electrochemical reaction to the bipolar plate, as well as high electrical conductivity for extracting generated electric current, and electrode substrates composed of carbon fibers and the like are widely used.
However, the following problems are known: (1) when the polymer electrolyte fuel cell is operated at a relatively low temperature of below 70° C. in a high current density region, as a result of blockage of the electrode substrate by liquid water generated in a large amount and shortage in the fuel gas supply, the fuel cell performance is impaired (this problem is hereinafter referred to as “flooding”); and (2) when the polymer electrolyte fuel cell is operated at a relatively low temperature of below 70° C. in a high current density region, as a result of blockage of the gas flow channel (hereinafter referred to as flow channel) of the bipolar plate by liquid water generated in a large amount and shortage in the fuel gas supply, the fuel cell performance is instantaneously impaired (this problem is hereinafter referred to as “plugging”). In order to solve these problems of (1) to (2), various efforts have been made.
Patent Document 1 suggests a gas diffusion electrode substrate in which a part of a microporous layer is impregnated in the inside of the electrode substrate. According to the fuel cell using this gas diffusion electrode substrate, the gas diffusion electrode substrate surface on the bipolar plate side is smooth and has high hydrophobicity, whereby liquid water is unlikely to stay in the flow channel, and plugging is improved. However, since the porosity inside the electrode substrate is reduced, there has been a problem that gas diffusivity is decreased and the fuel cell performance is deteriorated.
Patent Document 2 suggests a gas diffusion electrode substrate using FEP as the fluororesin of the electrode substrate. According to the fuel cell using this gas diffusion electrode substrate, FEP covers the carbon fiber of the electrode substrate, thus the gas diffusion electrode substrate surface on the bipolar plate side has high hydrophobicity, liquid water is unlikely to stay in the flow channel, and plugging is improved. However, since FEP covers the carbon fiber of the electrode substrate, there has been a problem that the interface resistance between the bipolar plate and the gas diffusion electrode substrate is increased.
Patent Document 3 suggests a fuel cell using a gas diffusion electrode substrate on which a microporous layer comprising carbon black and a fluororesin is formed in both sides of the electrode substrate. According to the fuel cell using this gas diffusion electrode substrate, the microporous layer on the bipolar plate side is smooth and has high hydrophobicity, whereby liquid water is unlikely to stay in the flow channel, and plugging is improved. However, since water removal from the electrode substrate to the bipolar plate is inhibited by the microporous layer on the bipolar plate side, there has been a problem that flooding is remarkable.
As described above, a variety of efforts have been made; however, one that can be satisfied as a gas diffusion electrode substrate which is excellent in its anti-flooding and anti-plugging characteristics, and also has excellent mechanical properties, electrical conductivity and thermal conductivity is yet to be discovered.